The present invention relates to a laser scanning unit (LSU) with multiple light beams, especially to a LSU having a Micro Electronic Mechanical System oscillatory mirror set composed of vertically stacked Micro Electronic Mechanical System (MEMS) arrays and a linear corresponding scanning lens set consisting of a plurality of F-Sin θ lens stacked vertically for minimizing volume of color printers and improving scanning efficiency.
There are many patents disclosing applied techniques in connection with laser beam printer (LBP) such as U.S. Pat. Nos. 5,128,795, 5,162,938, 5,329,399, 5,710,654, 5,757,533, 5,619,362, 5,721,631, 5,553,729, 5,111,219, 5,995,131, 6,724,509 and Japanese patent Nos. 4-50908 and 5-45580. A conventional Laser Beam Printer includes a laser scanning unit that uses a laser diode as a light source. A light beam emitted from the laser diode passes a collimator lens and becomes a parallel light bean, then through a cylinder lens for converging the light beam that passes along Y axis of a sub-major scanning direction, and directly transmits the light beams along X-axis of a main scanning direction so as to form a line image. The laser scanning unit also includes a polygonal mirror that is adapted to rotate at high speed so that a plurality of reflection mirrors uniformly and continuously arranged on the polygonal mirror are just located at or in the vicinity of a focal point of the above-mentioned line image. The polygon mirror that deflects the at least one light beam is disposed near or in focus of above image for controlling projecting direction of the laser beam so as to make the laser beam move parallel to X-axis of the main scanning direction in a constant angular velocity, and then being deflected to a fθ lens. The fθ lens is located at one side of the polygonal mirror and it can be a single-element scanning lens, or a two-element scanning lens, as disclosed in U.S. Pat. No. 5,995,131. The fθ lens projects the light beams deflected from the polygon mirror onto an image plane or a photoreceptor drum so as to achieve requirement of scanning linearity.
However, the above-described conventional laser scanning unit has the following disadvantages:
(1) The rotary polygonal mirror in the conventional laser scanning unit is very difficult to make and is with high manufacturing cost. Thus the cost of the laser scanning unit can't be reduced.
(2) Generally, the reflection mirrors on the polygonal mirror have a very small mirror width in the direction of Y-axis. Thus it is necessary to add a cylindrical lens inside the conventional laser scanning unit so as to make laser beams through the cylindrical lens be focused to form a line (or a point on Y-axis) before being projected onto the reflection mirrors of the polygonal mirror. Therefore, the conventional laser scanning unit has increased number of elements and requires increased assembling operations.
(3) The conventional polygonal mirror generates relatively high noises and it takes relatively long waiting period for the polygonal mirror to reach a working rotational speed after the laser scanning unit being initiated.
(4) When designing the fθ lens of conventional LSU, it is necessary to consider a deviation from the axis of the polygonal mirror. Thus, it is more difficult in design and manufacturing of the fθ lens.
Moreover, the laser scanning unit applied in conventional Laser Beam Printers needs to control reflective direction of a plurality of (such as four) laser beams for achieving requirement of scanning linearity synchronously. Examples of such patents include U.S. Pat. Nos. 6,798,820, 6,839,074 and 6,914,705. However, above LUS modules disclosed still use a polygonal mirror rotating at high speed to control reflection direction of multiple laser beams so that they not only have above disadvantages mentioned above but also have more complicated structure and disposition. This increases difficulties in design tasks and volume of color printers. Therefore, the requirements of compact size and light weight can't be met.